I think I've got a compiler which is functional enough to do some work. The next step is to make sure the installation works and is useful. I'm realizing it's been a really long time since I last did a full GCC build, so I'm recording the steps I used for later:

GAS:
$ ./configure --target tms9900

GCC:
$ ./configure --program-prefix=tms9900 --prefix /home/eric/dev/tios/toolchain/bin --target=tms9900 --enable-languages=c
$ make all-gcc
$ make install